EQUILIBRIUM IN THE REACTION BETWEEN NICKEL OR GALLIUM-IN-NICKEL SOLID SOLUTIONS AND CARBON MONOOXIDE

Abstract

The purpose of this work was to investigate the heterogeneous equilibrium: Ni(S) + 4CO(V) Ni(CO)4(V). The data on the temperature dependence of this reaction equilibrium constant enables to measure important thermodynamic parameters (such as reaction enthalpy and entropy). Pure nickel or the solid solutions of gallium in nickel were used as equilibrium solid phases. It should be stressed that only nickel and nickel alloys form the stable carbonyl compound (Ni(CO)4) in the reaction between solid metals and carbon monoxide at low vapor pressures (up to 1 bar). The considered reaction has high selectivity and in the case of nickel alloys only nickel (as a component) reacts with carbon monoxide. High temperature spectrophotometry is the main method of vapor-pressure measurement in this work. Nickel carbonyl vapor has the speci c bands in the near-UV absorption spectrum. The characteristic spectrum consists of main peak at 206 nm and two shoulders at 230 nm and 270 nm. The investigation of the optical absorption at these bands as a function of temperature allows one to determine the temperature dependence of nickel carbonyl concentration and, therefore, the equilibrium constant and some thermodynamic values for very reaction. To record the spectra during heating the experiment was carried out in an evacuated quartz cuvette. This cuvette was lled with nickel (nickel alloy) powder and with carbon monoxide gas. The vapor pressure was measured by the digital manometer (XP2i). Then the cuvette was sealed and annealed in the liquid thermostat for some days to reach an equilibrium state. After anneal the cuvette was quenched and analyzed for Ni(CO)4 concentration by spectral scanning. To nd the dependency between the optical absorption coef cients k and the Ni(CO)4 concentration in the gas phase the separate experiments for the spectrum scanning of carbonyl saturated vapor (Ni(CO)4 (S)Ni(CO)4 (V); Ni(CO)4 (L)Ni(CO)4 (V)) were organized at very low temperatures (-70 – -25 °C). Appreciable amounts of the nickel carbonyl were detected only at low temperatures (<130 °C), however at temperatures lower than 70 °C the reaction rate was very low. For the reaction with pure nickel the calculated value of the reaction entropy is –60.27±10.23 J/mol·K and enthalpy is–168.12±3.97 J/mol. Concerning the solid solutions of gallium in nickel, the prompt decrease of the equilibrium gas-phase carbonyl concentration with the increase of gallium concentration in the contacting solid solution was found.

Keywords

• vapor-pressure methods
• spectrophotometry
• heterogeneous equilibrium
• vapor phase
• absorption
• nickel carbonyl.